Various methods have been known as methods for producing cyclohexene. Partial hydrogenation of benzene is mentioned as one of them. Partial hydrogenation of benzene is generally performed by introducing hydrogen into a suspension of water and benzene in the presence of a ruthenium catalyst to allow a reaction to occur. A mixture containing water, cyclohexane, cyclohexene, unreacted benzene, and the like is obtained as a result of the reaction. In order to obtain high purity cyclohexene from this mixture, a method for separating cyclohexene from the reaction mixture poses a challenge. Since cyclohexane, cyclohexene, and benzene have close boiling points to each other, it is difficult to obtain high purity cyclohexene by the conventional distillation method. For this reason, an extractive distillation process using a solvent such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, adiponitrile, sulfolane, dimethyl malonate, dimethyl succinate, or ethylene glycol is proposed as a method for separating high purity cyclohexene from the above mixture (Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Laid-Open No. 9-169669    Patent Document 2: Japanese Patent Laid-Open No. 9-30994